In some multiplex transmission systems, a plurality of multiplex nodes each having a communication control circuit therein are connected to one another by means of a common signal transmission line (data bus) formed of a twisted pair wire, thus constituting a looped multiplex communication network (hereinafter referred to as "network") for multiplex transmission of data signals. Further, there are developed versions of these multiplex transmission systems in which a plurality of networks, having the aforesaid configuration and various response characteristics (e.g., different transmission speeds), are connected by means of a transmission control section (gateway node) which has a gateway function based on a CPU, whereby multiplex transmission is effected. The communication control circuit is formed of a microprocessor (CPU) for effecting electronic process control, a multiplex transmission control IC for effecting network transmission control, a buffer and a interface for sending and receiving signals, etc.
In one such multiplex transmission system, the gateway node receives transmit data from a multiplex node (sending multiplex node) which is connected to one network. If the gateway node concludes that the transmit data is a data signal to be subjected to a gateway function, moreover, it quickly sends this data signal to another network.
When a request-to-send is addressed from a receiving multiplex node connected to the one network to a sending multiplex node connected to the other network, the gateway node transmits this request-to-send to a relevant sending multiplex node. When return data corresponding to the request-to-send is sent from this sending multiplex node, the gateway node fetches this return data, and then transmits this return data to the receiving multiplex node.
When a new multiplex node is connected to the one network so that data from this multiplex node is transmitted to the other network, in this multiplex transmission system, however, it is necessary to interrupt the use of the multiplex transmission system and modify the design of a complicated data conversion operation of the gateway node. Thus, the modification of the design of the data conversion operation takes time and prohibits the use of the system in the meantime, and besides, entails higher manufacturing cost.
In the case where a multiplex node in the other network needs just a small part of data subjected to multiplex transmission by means of the one network, the gateway node must use its gateway function for all the transmitted data. Accordingly, the amount of data processing in the multiplex node in the other network increases, as well as the data traffic in the other network increases.